STRUCTURE SENSITIVE HYDROGEN ADSORPTION - EFFECT OF AG ON RU SIO2 CATALYSTS/

The dynamics and energetics of hydrogen chemisorption on silica-suppor ted ruthenium and ruthenium-silver bimetallic catalysts were studied b y H-1 NMR spectroscopy and microcalorimetry. It was observed that the amount of hydrogen adsorbed on Ru particles having intermediate and lo w heats of adsorption was significantly reduced with increasing amount s of silver. The desorption and adsorption rate constants, determined from selective excitation NMR experiments, were lower on Ru-Ag catalys ts than those on Ru catalysts at the same temperature and hydrogen cov erage. The apparent sticking coefficients of hydrogen on a Ru-Ag catal yst with 10 at% Ag were more than 10 times lower than those on a Ru ca talyst and were comparable to sticking coefficients reported in the li terature for hydrogen adsorbing on Ru single crystal. Thus silver was found to greatly affect both the dynamics and energetics of hydrogen c hemisorption on Ru/SiO2. However, ensemble and electronic effects did not play any role in causing these effects. It is postulated here that the influence of silver was due to its tendency to selectively segreg ate to edge, corner, and other low coordination structures on the Ru p article surface. Hence, hydrogen adsorption on these surfaces was foun d to be structure sensitive. (C) 1998 Academic Press.